1. Field of the Invention
The present invention relates to a method for manufacturing a solid electrolytic capacitor with conductive polymer as a solid electrolyte.
2. Background Art
As electronic equipment is becoming increasingly sophisticated, solid electrolytic capacitors used in it are also being required to have high performance. It is now essential for solid electrolytic capacitors to reduce the equivalent series resistance (hereinafter, ESR) in a high frequency range, especially in the field of computers and digital home appliances. Therefore, various measures have been proposed to achieve solid electrolytic capacitors with low ESR and still high capacity to correspond to the increasing current of electronic equipment in recent years.
FIG. 5 is an enlarged sectional view of a capacitor element of a conventional solid electrolytic capacitor. The capacitor element includes anode body 42, dielectric oxide film layer 44 formed thereon, and solid electrolyte 46 further formed thereon.
Anode body 42, which is a porous body formed by either etching valve metal foil or sintering valve metal powder, has fine pores 43 having an average pore diameter of about 0.1 μm to 0.8 μm.
Dielectric oxide film layer 44 is formed in fine pores 43 of anode body 42 and on the surface of outer periphery 42A of anode body 42. Solid electrolyte 46 is formed of a highly conductive polymer such as polythiophene to reduce the ESR of the solid electrolytic capacitor.
One well-known method for forming solid electrolyte 46 of polythiophene on the surface of dielectric oxide film layer 44 is a chemical polymerization method. In the method, either a thiophene monomer or a thiophene derivative monomer is subjected to a redox reaction with an oxidizing agent. The oxidizing agent can be applied using one of two well-known approaches. One is to mix it with the monomer in a solution, and the other is to previously form it as a layer thereof on dielectric oxide film layer 44.
In the chemical polymerization by the former approach, the monomer and the oxidizing agent are mixed in a solution, which means that the chemical polymerization is performed in the solution. This causes a conductive polymer such as poly(3,4-ethylenedioxythiophene) (hereinafter, PEDT) to be generated and float in the mixed solution, thus reducing the amount of PEDT formed on dielectric oxide film layer 44, and hence, reducing the yield.
As another problem of the former approach, the chemical polymerization reaction on outer periphery 42A of anode body 42 proceeds very fast. As a result, the PEDT clogs openings 43A of fine pores 43 as shown in FIG. 5 and is not formed very uniformly inside fine pores 43. This results in a decrease in capacitance.
In the chemical polymerization by the latter approach, on the other hand, a reaction solution is prepared containing, for example, 3,4-ethylenedioxythiophene (hereinafter, EDT) as a monomer, benzenesulfonic acid, and methanol. Meanwhile, a manganese dioxide layer is previously formed as an oxidizing agent layer on dielectric oxide film layer 44. Then, a reaction solution is brought into contact with the manganese dioxide formed on dielectric oxide film layer 44. In this approach, however, the chemical polymerization reaction stops halfway, making it impossible to form the PEDT stably on dielectric oxide film layer 44.